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    <title>如何提高flutter应用程序的性能 | 早起的年轻人</title>
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    <li><a href="#不要将小部件拆分为方法">不要将小部件拆分为方法</a></li>
    <li><a href="#避免重复重建所有小部件">避免重复重建所有小部件</a></li>
    <li><a href="#尽可能使用-const-小部件">尽可能使用 const 小部件</a></li>
    <li><a href="#使用itemextentlistview中的长列表">使用<code>itemExtent</code>ListView中的长列表</a></li>
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        <li><a href="#1---使用-fadetransition">1 - 使用 <code>FadeTransition</code></a></li>
        <li><a href="#2---使用-animatedopacity">2 - 使用 <code>AnimatedOpacity</code></a></li>
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    <article class="post">
        <header>
            <h1 class="post-title">如何提高flutter应用程序的性能</h1>
        </header>
        <date class="post-meta meta-date">
            2021年9月8日
        </date>
        
        <div class="post-meta">
            <span>|</span>
            
            <span class="meta-category"><a href='/categories/flutter'>flutter</a></span>
            
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            <span id="busuanzi_container_page_pv">|<span id="busuanzi_value_page_pv"></span><span>
                    阅读</span></span>
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        <div class="post-content">
            <blockquote>
<p>本文主要介绍</p>
</blockquote>
<p>关于如何提高 Flutter 应用程序的性能，存在许多疑问和问题。</p>
<p>我们必须澄清一下，Flutter 默认是高性能的，但我们在编写代码时必须避免犯一些错误，以使应用程序以出色且快速的方式运行。</p>
<p>通过我将给您的这些技巧，我们可以避免使用分析工具。</p>
<p>那么，让我们开始吧，但首先，让我们知道<strong>您在构建应用程序时的主要困难</strong>是<strong>什么？</strong></p>
<p><a href="https://codemagic.io/start/?utm_source=maintenance&amp;utm_medium=main_difficulty&amp;utm_campaign=blog_question">维护</a></p>
<p><a href="https://codemagic.io/start/?utm_source=no_mac&amp;utm_medium=main_difficulty&amp;utm_campaign=blog_question">我没有 mac</a></p>
<p><a href="https://codemagic.io/start/?utm_source=releasing&amp;utm_medium=main_difficulty&amp;utm_campaign=blog_question">释放</a></p>
<p><a href="https://codemagic.io/start/?utm_source=team_in_loop&amp;utm_medium=main_difficulty&amp;utm_campaign=blog_question">保持团队循环</a></p>
<h2 id="不要将小部件拆分为方法">不要将小部件拆分为方法</h2>
<p>当我们有一个很大的布局时，我们通常做的就是使用每个小部件的方法进行拆分。</p>
<p>以下示例是一个包含页眉、中心和页脚小部件的小部件。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MyHomePage</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  Widget _buildHeaderWidget() {
    <span style="color:#66d9ef">final</span> size <span style="color:#f92672">=</span> <span style="color:#ae81ff">40.0</span>;
    <span style="color:#66d9ef">return</span> Padding(
      padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">8.0</span>),
      child: CircleAvatar(
        backgroundColor: Colors.grey[<span style="color:#ae81ff">700</span>],
        child: FlutterLogo(
          size: size,
        ),
        radius: size,
      ),
    );
  }

  Widget _buildMainWidget(BuildContext context) {
    <span style="color:#66d9ef">return</span> Expanded(
      child: Container(
        color: Colors.grey[<span style="color:#ae81ff">700</span>],
        child: Center(
          child: Text(
            <span style="color:#e6db74">&#39;Hello Flutter&#39;</span>,
            style: Theme.of(context).textTheme.display1,
          ),
        ),
      ),
    );
  }

  Widget _buildFooterWidget() {
    <span style="color:#66d9ef">return</span> Padding(
      padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">8.0</span>),
      child: Text(<span style="color:#e6db74">&#39;This is the footer &#39;</span>),
    );
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      body: Padding(
        padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">15.0</span>),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            _buildHeaderWidget(),
            _buildMainWidget(context),
            _buildFooterWidget(),
          ],
        ),
      ),
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>我们之前看到的是一种反模式。内部发生的事情是，当我们对整个小部件进行更改和刷新时，它也会刷新我们在方法中拥有的小部件，这导致我们浪费 CPU 周期。</p>
<p>我们应该做的是通过以下方式将这些方法转换为无状态的Widget。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MyHomePage</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      body: Padding(
        padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">15.0</span>),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            HeaderWidget(),
            MainWidget(),
            FooterWidget(),
          ],
        ),
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">HeaderWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">final</span> size <span style="color:#f92672">=</span> <span style="color:#ae81ff">40.0</span>;
    <span style="color:#66d9ef">return</span> Padding(
      padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">8.0</span>),
      child: CircleAvatar(
        backgroundColor: Colors.grey[<span style="color:#ae81ff">700</span>],
        child: FlutterLogo(
          size: size,
        ),
        radius: size,
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MainWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Expanded(
      child: Container(
        color: Colors.grey[<span style="color:#ae81ff">700</span>],
        child: Center(
          child: Text(
            <span style="color:#e6db74">&#39;Hello Flutter&#39;</span>,
            style: Theme.of(context).textTheme.display1,
          ),
        ),
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">FooterWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Padding(
      padding: <span style="color:#66d9ef">const</span> EdgeInsets.all(<span style="color:#ae81ff">8.0</span>),
      child: Text(<span style="color:#e6db74">&#39;This is the footer &#39;</span>),
    );
  }
}
</code></pre></div><p>有状态/无状态小部件具有特殊的“缓存”机制(基于键、小部件类型和属性)，仅在必要时重建。除此之外，这有助于我们封装和重构我们的小部件。(分而治之)</p>
<p>添加<code>const</code>到我们的小部件是一个好主意，我们稍后会看到为什么这很重要。</p>
<h2 id="避免重复重建所有小部件">避免重复重建所有小部件</h2>
<p>这是我们开始使用 Flutter 时犯的一个典型错误，我们学会了重建我们的<code>StatefulWidget</code>using <code>setState</code>.</p>
<p>下面的示例是一个小部件，它包含一个中心的 Square，带有一个 fav 按钮，每次按下该按钮都会导致颜色发生变化。哦，该页面还有一个带有背景图像的小部件。</p>
<p>此外，我们将在每个小部件的构建方法之后添加一些打印语句，以查看它是如何工作的。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  Color _currentColor <span style="color:#f92672">=</span> Colors.grey;

  Random _random <span style="color:#f92672">=</span> <span style="color:#66d9ef">new</span> Random();

  <span style="color:#66d9ef">void</span> _onPressed() {
    <span style="color:#66d9ef">int</span> randomNumber <span style="color:#f92672">=</span> _random.nextInt(<span style="color:#ae81ff">30</span>);
    setState(() {
      _currentColor <span style="color:#f92672">=</span> Colors.primaries[randomNumber <span style="color:#f92672">%</span> Colors.primaries.length];
    });
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `MyHomePage`&#39;</span>);
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: Container(
              height: <span style="color:#ae81ff">150</span>,
              width: <span style="color:#ae81ff">150</span>,
              color: _currentColor,
            ),
          ),
        ],
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">BackgroundWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `BackgroundWidget`&#39;</span>);
    <span style="color:#66d9ef">return</span> Image.network(
      <span style="color:#e6db74">&#39;https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg&#39;</span>,
      fit: BoxFit.cover,
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>您将看到打印了 2 个日志：</p>
<pre><code>flutter: building `MyHomePage`
flutter: building `BackgroundWidget`
</code></pre><p>每次按下按钮时，我们都会刷新整个屏幕、Scaffold、Background 小部件，最后是对我们重要的容器。</p>
<p>随着我们了解更多，我们知道重建整个小部件不是一个好的做法，我们只需要重建我们想要更新的内容。</p>
<p>许多人知道这可以通过像<code>flutter_bloc</code>, <code>mobx</code>,<code>provider</code>等这样的状态管理包来完成。但很少有人知道它也可以在没有任何外部包的情况下完成，也就是说，使用 Flutter 框架已经提供的开箱即用的类。</p>
<p>我们现在将使用 <code>ValueNotifier.</code></p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  <span style="color:#66d9ef">final</span> _colorNotifier <span style="color:#f92672">=</span> ValueNotifier<span style="color:#f92672">&lt;</span>Color<span style="color:#f92672">&gt;</span>(Colors.grey);
  Random _random <span style="color:#f92672">=</span> <span style="color:#66d9ef">new</span> Random();

  <span style="color:#66d9ef">void</span> _onPressed() {
    <span style="color:#66d9ef">int</span> randomNumber <span style="color:#f92672">=</span> _random.nextInt(<span style="color:#ae81ff">30</span>);
    _colorNotifier.value <span style="color:#f92672">=</span>
        Colors.primaries[randomNumber <span style="color:#f92672">%</span> Colors.primaries.length];
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> dispose() {
    _colorNotifier.dispose();
    <span style="color:#66d9ef">super</span>.dispose();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `MyHomePage`&#39;</span>);
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: ValueListenableBuilder(
              valueListenable: _colorNotifier,
              builder: (_, value, __) <span style="color:#f92672">=&gt;</span> Container(
                height: <span style="color:#ae81ff">150</span>,
                width: <span style="color:#ae81ff">150</span>,
                color: value,
              ),
            ),
          ),
        ],
      ),
    );
  }
}
</code></pre></div><p>现在您将看不到任何打印语句。</p>
<p>这很完美，我们只是重建我们需要的小部件。如果我们想要更好地分离业务逻辑和视图(并且可能在其中添加更多逻辑)，我们还可以使用另一个有趣的小部件，我们还可以在通知程序中处理更多数据。</p>
<p>现在使用相同的示例<code>ChangeNotifier</code>。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#75715e">//------ ChangeNotifier class ----//
</span><span style="color:#75715e"></span><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MyColorNotifier</span> <span style="color:#66d9ef">extends</span> ChangeNotifier {
  Color myColor <span style="color:#f92672">=</span> Colors.grey;
  Random _random <span style="color:#f92672">=</span> <span style="color:#66d9ef">new</span> Random();

  <span style="color:#66d9ef">void</span> changeColor() {
    <span style="color:#66d9ef">int</span> randomNumber <span style="color:#f92672">=</span> _random.nextInt(<span style="color:#ae81ff">30</span>);
    myColor <span style="color:#f92672">=</span> Colors.primaries[randomNumber <span style="color:#f92672">%</span> Colors.primaries.length];
    notifyListeners();
  }
}
<span style="color:#75715e">//------ State class ----//
</span><span style="color:#75715e"></span>
<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  <span style="color:#66d9ef">final</span> _colorNotifier <span style="color:#f92672">=</span> MyColorNotifier();

  <span style="color:#66d9ef">void</span> _onPressed() {
    _colorNotifier.changeColor();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> dispose() {
    _colorNotifier.dispose();
    <span style="color:#66d9ef">super</span>.dispose();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `MyHomePage`&#39;</span>);
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: AnimatedBuilder(
              animation: _colorNotifier,
              builder: (_, __) <span style="color:#f92672">=&gt;</span> Container(
                height: <span style="color:#ae81ff">150</span>,
                width: <span style="color:#ae81ff">150</span>,
                color: _colorNotifier.myColor,
              ),
            ),
          ),
        ],
      ),
    );
  }
}
</code></pre></div><p>最重要的是，通过这最后 2 个小部件，我们可以避免不必要的重建。</p>
<h2 id="尽可能使用-const-小部件">尽可能使用 const 小部件</h2>
<p>将关键字<code>const</code>用于我们可以在编译时初始化的常量是一种很好的做法。让我们也不要忘记<code>const</code>尽可能多地使用我们的小部件，这使我们能够捕获和重用小部件，以避免由其祖先引起的不必要的重建。</p>
<p>让我们使用 重用最后一个示例<code>setState</code>，但现在我们将添加一个计数器，每次按下 fav 按钮时，该计数器都会增加值。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  <span style="color:#66d9ef">int</span> _counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() {
    setState(() {
      _counter<span style="color:#f92672">++</span>;
    });
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `MyHomePage`&#39;</span>);
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
              child: Text(
            _counter.toString(),
            style: Theme.of(context).textTheme.display4.apply(
                  color: Colors.white,
                  fontWeightDelta: <span style="color:#ae81ff">2</span>,
                ),
          )),
        ],
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">BackgroundWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `BackgroundWidget`&#39;</span>);
    <span style="color:#66d9ef">return</span> Image.network(
      <span style="color:#e6db74">&#39;https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg&#39;</span>,
      fit: BoxFit.cover,
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>我们有 2 个打印语句，一个在主小部件的构建中，另一个在后台小部件中。每次我们按下 fab 按钮时，我们都会看到子部件也被重建，即使它有固定的内容。</p>
<pre><code>flutter: building `MyHomePage`
flutter: building `BackgroundWidget`
</code></pre><p>现在<code>const</code>用于我们的小部件，代码如下所示：</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  <span style="color:#66d9ef">int</span> _counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() {
    setState(() {
      _counter<span style="color:#f92672">++</span>;
    });
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `MyHomePage`&#39;</span>);
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: <span style="color:#66d9ef">const</span> BackgroundWidget(),
          ),
          Center(
              child: Text(
            _counter.toString(),
            style: Theme.of(context).textTheme.display4.apply(
                  color: Colors.white,
                  fontWeightDelta: <span style="color:#ae81ff">2</span>,
                ),
          )),
        ],
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">BackgroundWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#66d9ef">const</span> BackgroundWidget();

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `BackgroundWidget`&#39;</span>);
    <span style="color:#66d9ef">return</span> Image.network(
      <span style="color:#e6db74">&#39;https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg&#39;</span>,
      fit: BoxFit.cover,
    );
  }
}
</code></pre></div><p>当我们在按下 fav 按钮时看到日志时，我们只能看到父小部件的初始打印(当然我们可以使用我在本技巧之前向您展示的技术改进这一点)，因此我们避免了标记为的小部件的重建常量</p>
<h2 id="使用itemextentlistview中的长列表">使用<code>itemExtent</code>ListView中的长列表</h2>
<p>有时候当我们有一个很长的列表，想用scroll进行大幅度的跳转时， 的使用<code>itemExtent</code>很重要，我们来看一个简单的例子。</p>
<p>我们有一个包含 10,000 个元素的列表。按下按钮后，我们将跳转到最后一个元素。在这个例子中，我们不会使用<code>itemExtent</code>，我们会让孩子们决定它的大小。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MyHomePage</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#66d9ef">final</span> widgets <span style="color:#f92672">=</span> List.generate(
    <span style="color:#ae81ff">10000</span>,
    (index) <span style="color:#f92672">=&gt;</span> Container(
      height: <span style="color:#ae81ff">200.0</span>,
      color: Colors.primaries[index <span style="color:#f92672">%</span> Colors.primaries.length],
      child: ListTile(
        title: Text(<span style="color:#e6db74">&#39;Index: </span><span style="color:#e6db74">$</span>index<span style="color:#e6db74">&#39;</span>),
      ),
    ),
  );

  <span style="color:#66d9ef">final</span> _scrollController <span style="color:#f92672">=</span> ScrollController();

  <span style="color:#66d9ef">void</span> _onPressed() <span style="color:#66d9ef">async</span> {
    _scrollController.jumpTo(
      _scrollController.position.maxScrollExtent,
    );
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: ListView(
        controller: _scrollController,
        children: widgets,
      ),
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>如果您看到结果，则跳转非常缓慢(约 10 秒)。这是因为让孩子们决定自己的尺寸的成本。这甚至会阻塞用户界面！</p>
<p>为了避免这种情况，我们必须使用<code>itemExtent</code>属性，使用此滚动机制可以利用子项范围的预知来节省工作。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">MyHomePage</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#66d9ef">final</span> widgets <span style="color:#f92672">=</span> List.generate(
    <span style="color:#ae81ff">10000</span>,
    (index) <span style="color:#f92672">=&gt;</span> Container(
      color: Colors.primaries[index <span style="color:#f92672">%</span> Colors.primaries.length],
      child: ListTile(
        title: Text(<span style="color:#e6db74">&#39;Index: </span><span style="color:#e6db74">$</span>index<span style="color:#e6db74">&#39;</span>),
      ),
    ),
  );

  <span style="color:#66d9ef">final</span> _scrollController <span style="color:#f92672">=</span> ScrollController();

  <span style="color:#66d9ef">void</span> _onPressed() <span style="color:#66d9ef">async</span> {
    _scrollController.jumpTo(
      _scrollController.position.maxScrollExtent,
    );
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: ListView(
        controller: _scrollController,
        children: widgets,
        itemExtent: <span style="color:#ae81ff">200</span>,
      ),
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>有了这个小小的改变，我们就可以立即跳跃，没有延迟。</p>
<h2 id="避免在-animatedbuilder-中重建不必要的小部件">避免在 AnimatedBuilder 中重建不必要的小部件</h2>
<p>通常我们想为我们的小部件添加动画。然后我们通常做的是向我们添加一个侦听器<code>AnimationController</code>并调用<code>setState</code>. 但是正如我们在开始时看到的那样，这不是一个好的做法。相反，我们将使用<code>AnimatedBuilder</code>小部件仅重建我们想要制作动画的小部件。</p>
<p>我们将创建一个屏幕，其中包含一个中心显示当前计数的小部件，当按下 fav 按钮时，小部件会旋转 360。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span>
    <span style="color:#66d9ef">with</span> SingleTickerProviderStateMixin {
  AnimationController _controller;
  <span style="color:#66d9ef">int</span> counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() {
    setState(() {
      counter<span style="color:#f92672">++</span>;
    });
    _controller.forward(from: <span style="color:#ae81ff">0.0</span>);
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> initState() {
    _controller <span style="color:#f92672">=</span> AnimationController(
        vsync: <span style="color:#66d9ef">this</span>, duration: <span style="color:#66d9ef">const</span> Duration(milliseconds: <span style="color:#ae81ff">600</span>));
    <span style="color:#66d9ef">super</span>.initState();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> dispose() {
    _controller.dispose();
    <span style="color:#66d9ef">super</span>.dispose();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        builder: (_, child) <span style="color:#f92672">=&gt;</span> Transform(
          alignment: Alignment.center,
          transform: Matrix4.identity()
            ..setEntry(<span style="color:#ae81ff">3</span>, <span style="color:#ae81ff">2</span>, <span style="color:#ae81ff">0.001</span>)
            ..rotateY(<span style="color:#ae81ff">360</span> <span style="color:#f92672">*</span> _controller.value <span style="color:#f92672">*</span> (pi <span style="color:#f92672">/</span> <span style="color:#ae81ff">180.0</span>)),
          child: CounterWidget(
            counter: counter,
          ),
        ),
      ),
    );
  }
}

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">CounterWidget</span> <span style="color:#66d9ef">extends</span> StatelessWidget {
  <span style="color:#66d9ef">final</span> <span style="color:#66d9ef">int</span> counter;

  <span style="color:#66d9ef">const</span> CounterWidget({Key key, <span style="color:#66d9ef">this</span>.counter}) <span style="color:#f92672">:</span> <span style="color:#66d9ef">super</span>(key: key);

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    print(<span style="color:#e6db74">&#39;building `CounterWidget`&#39;</span>);
    <span style="color:#66d9ef">return</span> Center(
      child: Text(
        counter.toString(),
        style: Theme.of(context).textTheme.display4.apply(fontWeightDelta: <span style="color:#ae81ff">3</span>),
      ),
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>完美，我们已经拥有了。现在让我们在计数器小部件中添加一个打印件，看看会发生什么，然后再试一次。</p>
<pre><code>flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
...
flutter: building `CounterWidget`
</code></pre><p>我们看到在旋转时它正在重建我们的小部件。但是，有很多打印语句，我们如何避免呢？(可以只重建一次，因为我们使用 setState 来刷新计数器)。</p>
<p>很简单，我们使用<code>child</code>提供的属性<code>AnimatedBuilder</code>，它允许我们缓存小部件以在我们的动画中重用它们。我们这样做是因为那个小部件不会改变，它唯一会做的就是旋转，但为此我们有<code>Transform</code>小部件。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart">  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        child: CounterWidget(
          counter: counter,
        ),
        builder: (_, child) <span style="color:#f92672">=&gt;</span> Transform(
          alignment: Alignment.center,
          transform: Matrix4.identity()
            ..setEntry(<span style="color:#ae81ff">3</span>, <span style="color:#ae81ff">2</span>, <span style="color:#ae81ff">0.001</span>)
            ..rotateY(<span style="color:#ae81ff">360</span> <span style="color:#f92672">*</span> _controller.value <span style="color:#f92672">*</span> (pi <span style="color:#f92672">/</span> <span style="color:#ae81ff">180.0</span>)),
          child: child,
        ),
      ),
    );
  }
</code></pre></div><p>我们有相同的视觉结果，但如果我们看到日志，它不再重建我们的计数器小部件(只有一次因为我们更新了计数器)，我们优化了动画。</p>
<h2 id="避免在动画中使用不透明度">避免在动画中使用不透明度</h2>
<p>我们将再次提及动画，因为很多时候我们通常会<code>Opacity</code>在其中使用小部件。</p>
<p>我们将重新创建之前的示例，但<code>Transform</code>我们将使用 Opacity 来使小部件消失并再次出现，而不是使用。</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span>
    <span style="color:#66d9ef">with</span> SingleTickerProviderStateMixin {
  AnimationController _controller;
  <span style="color:#66d9ef">int</span> counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() {
    setState(() {
      counter<span style="color:#f92672">++</span>;
    });
    _controller.forward(from: <span style="color:#ae81ff">0.0</span>);
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> initState() {
    _controller <span style="color:#f92672">=</span> AnimationController(
        vsync: <span style="color:#66d9ef">this</span>, duration: <span style="color:#66d9ef">const</span> Duration(milliseconds: <span style="color:#ae81ff">600</span>));
    _controller.addStatusListener((status) {
      <span style="color:#66d9ef">if</span> (status <span style="color:#f92672">==</span> AnimationStatus.completed) {
        _controller.reverse();
      }
    });
    <span style="color:#66d9ef">super</span>.initState();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> dispose() {
    _controller.dispose();
    <span style="color:#66d9ef">super</span>.dispose();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        child: CounterWidget(
          counter: counter,
        ),
        builder: (_, child) <span style="color:#f92672">=&gt;</span> Opacity(
          opacity: (<span style="color:#ae81ff">1</span> <span style="color:#f92672">-</span> _controller.value),
          child: child,
        ),
      ),
    );
  }
}
</code></pre></div><p>
        <a data-fancybox="gallery" href="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif">
            <img class="mx-auto" alt="img" src="https://luckly007.oss-cn-beijing.aliyuncs.com/img/giphy.gif" />
        </a>
    </p>
<p>然而，正如我们看到的动画工作，动画<code>Opacity</code>小部件直接导致小部件(可能还有它的子树)重建每一帧，这不是很有效。</p>
<p>因此，为了改进和优化这一点，我们有两个选择：</p>
<h3 id="1---使用-fadetransition">1 - 使用 <code>FadeTransition</code></h3>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span>
    <span style="color:#66d9ef">with</span> SingleTickerProviderStateMixin {
  AnimationController _controller;
  <span style="color:#66d9ef">int</span> counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() {
    setState(() {
      counter<span style="color:#f92672">++</span>;
    });
    _controller.forward(from: <span style="color:#ae81ff">0.0</span>);
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> initState() {
    _controller <span style="color:#f92672">=</span> AnimationController(
        vsync: <span style="color:#66d9ef">this</span>, duration: <span style="color:#66d9ef">const</span> Duration(milliseconds: <span style="color:#ae81ff">600</span>));
    _controller.addStatusListener((status) {
      <span style="color:#66d9ef">if</span> (status <span style="color:#f92672">==</span> AnimationStatus.completed) {
        _controller.reverse();
      }
    });
    <span style="color:#66d9ef">super</span>.initState();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  <span style="color:#66d9ef">void</span> dispose() {
    _controller.dispose();
    <span style="color:#66d9ef">super</span>.dispose();
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: FadeTransition(
        opacity: Tween(begin: <span style="color:#ae81ff">1.0</span>, end: <span style="color:#ae81ff">0.0</span>).animate(_controller),
        child: CounterWidget(
          counter: counter,
        ),
      ),
    );
  }
}
</code></pre></div><h3 id="2---使用-animatedopacity">2 - 使用 <code>AnimatedOpacity</code></h3>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-dart" data-lang="dart"><span style="color:#66d9ef">const</span> duration <span style="color:#f92672">=</span> <span style="color:#66d9ef">const</span> Duration(milliseconds: <span style="color:#ae81ff">600</span>);

<span style="color:#66d9ef">class</span> <span style="color:#a6e22e">_MyHomePageState</span> <span style="color:#66d9ef">extends</span> State<span style="color:#f92672">&lt;</span>MyHomePage<span style="color:#f92672">&gt;</span> {
  <span style="color:#66d9ef">int</span> counter <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>;
  <span style="color:#66d9ef">double</span> opacity <span style="color:#f92672">=</span> <span style="color:#ae81ff">1.0</span>;

  <span style="color:#66d9ef">void</span> _onPressed() <span style="color:#66d9ef">async</span> {
    counter<span style="color:#f92672">++</span>;
    setState(() {
      opacity <span style="color:#f92672">=</span> <span style="color:#ae81ff">0.0</span>;
    });
    <span style="color:#66d9ef">await</span> Future.delayed(duration);
    setState(() {
      opacity <span style="color:#f92672">=</span> <span style="color:#ae81ff">1.0</span>;
    });
  }

  <span style="color:#960050;background-color:#1e0010">@</span>override
  Widget build(BuildContext context) {
    <span style="color:#66d9ef">return</span> Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedOpacity(
        opacity: opacity,
        duration: duration,
        child: CounterWidget(
          counter: counter,
        ),
      ),
    );
  }
}
</code></pre></div><p>这两个选项都比<code>Opacity</code>直接使用小部件更有效。</p>
<h2 id="概括">概括</h2>
<p>正如我最初提到的，Flutter 足够强大，可以毫无问题地运行我们的应用程序，但遵循良好实践并尽可能优化我们的应用程序总是好的。</p>
<p><a href="https://blog.codemagic.io/how-to-improve-the-performance-of-your-flutter-app./#dont-split-your-widgets-into-methods">https://blog.codemagic.io/how-to-improve-the-performance-of-your-flutter-app./#dont-split-your-widgets-into-methods</a></p>
        </div>

        
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        <li><strong>原文作者：</strong><a rel="author" href="https://luckly.work/">luckly</a></li>
        <li style="word-break:break-all"><strong>原文链接：</strong><a href="https://luckly.work/post/flutter_basic/%E5%A6%82%E4%BD%95%E6%8F%90%E9%AB%98flutter%E5%BA%94%E7%94%A8%E7%A8%8B%E5%BA%8F%E7%9A%84%E6%80%A7%E8%83%BD/">https://luckly.work/post/flutter_basic/%E5%A6%82%E4%BD%95%E6%8F%90%E9%AB%98flutter%E5%BA%94%E7%94%A8%E7%A8%8B%E5%BA%8F%E7%9A%84%E6%80%A7%E8%83%BD/</a></li>
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